Cut-off drains deliver rainwater run-off from roads onto farmland where it sometimes creates erosion and deep gullies. This potentially destructive practice can be changed to a gain for the farmers by diverting the water into ground tanks, small earth dams or land for seasonal irrigation.
Cut-off drain (c) E. Nissen-Petersen, Kenya
Cut-off drain to a pan (c) E. Nissen-Petersen, Kenya
Here a cut-off drain diverts run-off water from a road into a natural and shallow depression on the lower side of a road called a pan.
A pan can be made into a pond by deepening the reservoir and place the excavated soil as a dam wall (embankment) with two spillways (overflows) on the lower side of the reservoir. Ponds are small earth dams.
A borrow pit (c) E. Nissen-Petersen, Kenya
Where road contractors have excavated murram for road construction and left ‘borrow pits’ or ‘murram pits’, these can be converted into pans or ponds by digging a trench to divert water from a road into the pit.
Usually these pits have water-tight (impermeable) floors through which water cannot leak into the underground.
Charco dams (c) E. Nissen-Petersen, Kenya
Charco ponds and Charco dams are half-ball shaped (hemi-spherical) excavations where the soil is placed as a dam wall around the excavation, except at the inflow channel which has two spillways for safe discharge of surplus water. Charco ponds and dams are viable in flat land.
Hillside dam (c) E. Nissen-Petersen, Kenya
Hillside ponds/dams have a semi-circular dam wall made of the soil excavated for the water reservoir. A stony spillway is built onto each end of the dam wall. These dams are designed to be constructed on rolling land and hill sides.
Valley dam (c) E. Nissen-Petersen, Kenya
Valley dams are straight dam walls built across narrow points in valleys. A wide spillway lined with stones is built at each end of the dam wall to discharge surplus water safely.
Due to global warming, many valley dams have been damaged by extraordinarily big thunder storms exceeding the design criteria of the highest rainfall in the last 50 years.
Excavation dams (c) E. Nissen-Petersen, Kenya
Excavation dams should be circular or oval excavations where the excavated soil is used for building the dam walls whose sides should slope at least 45 degrees.
The excavation dam in the above photo was a waste of money and labour. The sides are too steep and therefore collapsing. The soil is too porous and can therefore not hold any water.
Excavation ponds (c) E. Nissen-Petersen, Kenya
The photo above shows a series of well designed and constructed excavation ponds have been filled with run-off water from roads. The embankments should be stabilised with grass.
A leaking water tank made of galvanized iron sheets (c) E. Nissen-Petersen, Kenya
Repaired oil drums (c) E. Nissen-Petersen, Kenya
A repaired tank (c) E. Nissen-Petersen, Kenya
Sketch of a watertank (c) Natural Farming Network (Vukasin et al. 1995)
Short description: This datasheet describes methods to safely and hygienically store collected water, as well as how to build and repair water tanks, oil drums and jerry cans.
Introduction
Rains produce
plenty of clean water running off roads, roofs, and rocks. This
rainwater can be stored for the dry seasons when it is needed most.
There are three types of storage, namely:
1) Storage in reservoirs, such as earth dams and ponds.
2) Storage in tanks
3) Storage in situ, such as in soil and sand.
Storage in reservoirs
If
farmers want to have water during dry seasons, they should ‘harvest’ it
during the annual four months with rain, just like Scandinavian farmers
harvest and store sufficient fodder for their livestock during six
months of summer to feed their livestock for the 6 winter months when
the animals are tied up in stable.
There are many types of structures suitable for surface storage of harvested rainwater but nearly all of them lose water in one way or another, such as:
1)Evaporation. In hot climates it amounts to about 3 mm/day = 90 cm in a month. The solution for water tanks is to roof them but since earth dams cannot be roofed the evaporation losses cannot be reduced.
2) Increased consumption of water from tanks situated next to a house can be caused by neighbours begging for water and children forgetting to close the water tap. This can be avoided by extending the draw-off pipe into the house and locking the water tap.
3) Animals breaking into fenced earth dams are difficult to prevent. A profitable solution could be to charge the livestock owners a fee for watering their animals.
4) Seepage through the floor of earth dams is reduced by silt brought in by rainy seasons but siltation reduces the storage capacity of earth dams.
These, and other, losses of water during storage should be considered when planning designing a water project for either water for domestic user, livestock or irrigation. For example, since about half of the water stored in earth dams will disappear due to evaporation and seepage, the reservoirs should be built to store double the volume of water required.
Storage in Tanks
An internal apron can be made with a short length of bamboo or an empty beer bottle (c) E. Nissen-Petersen, Kenya
General guidelines for water storage in tanks
Fresh or purified water can quickly become re-contaminated because:
1) The containers used to store the water are not clean.
2) Unclean things are dipped into the water (this includes hands, clothes, etc.).
3) The water is not covered and so insects, dust or other foreign substances can enter the water. Chemically disinfected water can have a residual protection which will deal with light recontamination, but even this protection disappears in time.
Thus, to prevent recontamination, only clean storage containers should be used and the water should be protected from any contact with objects other than the container.
The container requires periodic emptying, washing, and rinsing with scalding or heavily chlorinated water, to prevent the growth of biofilms.
The storage container should be equipped with a practical mechanism to retrieve the water, e.g. a tap (spigot) especially when bigger containers are used.Storing water for later use is more difficult than collecting water.
Ways to store water for household use include tanks or cisterns. Tanks can be constructed of bricks, masonry, corrugated steel sheets, or reinforced concrete, either above ground or below ground.
The capacity of the tanks should be determined based on the run-off expected and on the estimated daily use. Before the rain starts, the tank or storage area should be clean.
The first direct flush of rainwater should be directed away from the storage, since it contains the dirt from the catchment area. Cover the tank to prevent evaporation from the sun, keep the water surface clean, and prevent mosquitos from entering the water.
The storage tank should be placed near the place of usage, e.g. the kitchen. Furthermore, there should be a possibility to redirect the overflow or spilled water to a nearby garden or orchard. Storage tanks and reservoirs can become breeding places for malaria mosquitos.
The open water area can be used as breeding places for the mosquitos even in the dry season, when malaria transmission is normally decreased.
The open water surface should not be accessible to mosquitoes: the tanks should be covered and all other inlets (taps, ventilation pipes) screened with mosquito-proof mesh. It should also be avoided that breeding sites are established downstream of the overflow.
Storage in Situ
The
cheapest method of storing rainwater is to recharge shallow ground
water aquifers, also called in situ storage, during rainy seasons and
draw the water by means of hand-dug wells throughout the year. However,
this cheap method may not always succeed because:
1. The water may seep deep into the underground where it becomes salty and unfit for human consumption
2. The water may be too deep for shallow wells and require investment in expensive boreholes and pumps.
Agarwal, A. and Narain, S. (Eds) (2003). Dying wisdom. Rise, fall and potential of India’s traditional water harvesting systems. State of India’s Environment: a citizens’ report. Centre for Science and Environment, New Delhi. ISBN 81-86906-07-X www.cseindia.org
Anschuetz, J. and Nederlof, M. (Eds) (2001). Water harvesting and soil moisture retention. CTA Agrodok-series No.13. Technical Centre for Agricultural and Rural Cooperation ACP-EU. Agromisa Foundation ISBN 90-72746-75-9
Chleq, J.-L. and Dupriez, H. (1988). Vanishing Land and Water. Soil and water conservation in dry lands. Land and Life Macmillan Press Ltd, London. ISBN 0-333-44597-X
De Vrees, L. 1987. Rainwater tank programme. Machakos Diocese, Box 640, Machakos, Kenya.
Dupriez, H. and De Leener, P. (1992). Ways of Water: Run-off, Irrigation and Drainage. Land and Life Macmillan Press Ltd, London. ISBN 0-333-57078-2 (UK)
Enyatseng, G. (1998). Evaluation of ferrocement water tanks. Botswana Technology Centre, P/Bag 0082, Gaborone, Botswana.
Gould, J and Nissen-Petersen, E. (1999). Rainwater catchment systems for domestic supply. Intermediate Technology Publications, London, UK.
Gould, J. (1987). Assessment of roof and ground catchment systems in Botswana. 3rd IRSA Conference, Khon Kaen University, Thailand.
Gould, J. (1991). Rainwater catchment systems for household water supply. ENSIC Review, A.I.T. Bangkok, Thailand.
Gould, J. (1995). Development in rainwater catchment systems in eastern and southern Africa. 7th IRSA Conference, Beijing China.
Gould, J. (1998). Review of recent developments in rainwater catchment systems technology in eastern and southern Africa. Science, Vol. 16, No.1.
Gould, J. an . Intermediate Technology Publications Ltd, UK. ISBN 1 85339 456 4
Hasse, R. (1989). Rainwater Reservoirs above ground structures for roof catchments. GATE, Germany.
Hatibu, N., Mahoo, H. F. (Eds) (2000). Rainwater harvesting for natural resources management. A. planning guide for Tanzania. RELMA Technical handbook No.22. Sida Regional Land Management Unit, Nairobi, Kenya. ISBN 9966-896-52-X
Lee, M. and Nissen-Petersen, E. 1989. The use of low-cost self-help rainwater harvesting systems for community water supply in southern Kenya. 4th IRCA Conference, Manila, Philippines.
Lee, M. and Visschers, J.T. 1990. Water harvesting in five African countries. IRC, The Hague, Netherlands.
Malesu, M.M., Sang, J.K., Odhiambo, O.J., Oduor, A.R. and Nyabenge, M. (2006). Rainwater harvesting innovations in response to water scarcity: The Lare experience. Technical Report No.32. Regional Land Management Unit, Nairobi, Kenya. ISBN 92 9059 197 8
Ngigi, S.N. 2003. Rainwater Harvesting for Improved Food Security. Greater Horn of Africa Rainwater Partnership and Kenya Rainwater Association, Nairobi.
Ngigi, S.N. 2003. Rainwater Harvesting for Improved Food Security. Greater Horn of Africa Rainwater Partnership and Kenya Rainwater Association, Nairobi.
Nissen-Petersen, E. (1982). Rain Catchment and Water Supply in Rural Africa: A Manual. Hodder and Stoughton, Great Britain. ISBN 0340-28429-3.
Nissen-Petersen, E. (2006). Water from Dry Riverbeds. How dry and sandy riverbeds can be turned into water sources by hand-dug wells, subsurface dams, weirs and sand dams. Kenya.
Nissen-Petersen, E. (2006). Water from Roads. A handbook for technicians and farmers on harvesting rainwater from roads. Kenya.
Nissen-Petersen, E. (2006). Water from Rock Outcrops. A handbook for engineers and technicians on site investigations, designs, construction and maintenance of rock catchment tanks and dams. Kenya.
Nissen-Petersen, E. (2006). Water from Small Dams. A handbook for technicians, farmers and other on site investigations, designs, cost estimates, construction and maintenance of small earth dams. Kenya.
Nissen-Petersen, E. (2007). Water from Roofs. A handbook for technicians and builders on survey, design, construction and maintenance of roof catchments. Kenya.
Nissen-Petersen, E. 1982. Rain catchment and water supply in rural Africa. Hodder & Stoughton, London, UK.
Nissen-Petersen, E. 1990. Water tanks with guttering and hand-pump. Manual No. 1 of Harvesting Rainwater in Semi-arid Africa. Danida, Kenya
Nissen-Petersen, E. 1992. How to an underground tank with domes. ASALCON, Kenya.
Nissen-Petersen, E. 1992. How to build cylindrical tanks with domes. ASALCON, Kenya.
Nissen-Petersen, E. 1992. How to build smaller water tanks and jars. ASALCON, Kenya.
Nissen-Petersen, E. 1992. How to make and install gutters with splash-guard. ASALCON, Kenya.
Nissen-Petersen, E. 1992. How to repair various types of water tanks. ASALCON, Kenya.
Pacey, A. and Cullis, A. (1986). Rainwater Harvesting: the collection of rainfall and runoff in rural areas, IT Publications.
Pacey, A. and Cullis, A. (1986). Rainwater Harvesting: the collection of rainfall and runoff in rural areas,.IT Publications.
Teyssier, A. (2001). Establishing and managing waterpoints for village livestock. A guide for rural extension workers in the sudano-sahelian zone. CTA Agrodok-series No.27. Technical Centre for Agricultural and Rural Cooperation ACP-EU. Agromisa Foundation ISBN 9907246-90-2
Water, Engineering and Development Centre (WEDC) (1998). The worth of water: Technical briefs on health, water and sanitation. IT Publications, London. ISBN 1 85339 069 0.
Water, Engineering and Development Centre (WEDC) (1999). Running water: More technical briefs on health, water and sanitation. Edited by Rod Shaw. IT Publications, London. ISBN 1 85339 450 5.
Watt, S. 1978. Ferrocement water tanks and their construction. ITDG London, UK
The required storage capacity of a water reservoir depends on:
The daily required volumes and quality of water measured in litres.
The length of the dry seasons during which these volumes and quality of water are required.
Example on water demand for a homestead:
While the number of days in a dry season can be estimated fairly easy, such as 180 days without rain in a semi-arid region, the volume of water required for each of the 180 days can be calculated using the following guidelines on daily requirements of water for a rural homestead:
Water users
Daily requirements Litres
Number of days without rains
Required volume for a dry season Litres
1 person
15
180
2,700
1 grade cow
50
180
9,000
1 local cow
20
180
3,600
1 goat
5
180
900
1 sheep
5
180
900
1 hen
0.3
180
54
3.3 mm on 4048 m2 (1 acre) with drip irrigation
1,336
60 days x 1,336
80,150
5 mm on 4048 m2 (1 acre) with furrow irrigation
2,024
60 days x 2024
121,440
4.3 mm on 4048 m2 (1 acre) with sprinkler irrigation
1,741
60 days x 1741
104,460
(RELMA 2001, by I.V.Sijali)
A homestead with 6 persons, 4 local cows, 20 goats and sheep (shoats) and 20 hens who wants to irrigate 2023 m2 (1/4 acre) with drip irrigation requires the following volume of water for a 180 days dry season without any rains:
Litres
Clean water from roof for domestic use: 6 persons x 2,700 L
16,200
Unclean water from a water hole in a riverbed: 4 cows x 3,600 L
14,400
Unclean water from a water hole in a riverbed: 20 shoats x 900 L
18,000
Unclean water from a ground tank or a pond: 20 hens x 54 L
1,080
Unclean water from a ground tank or a pond for irrigating 1/4 acre: 80,150 x 1/4
20,038
Total storage requirement
69,718
Add 20% loss due to evaporation and seepage
13,944
Total storage requirement for a 180 day long dry period
83,662
This example shows that a rural homestead in a dry area could use 3 types of water sources:
1) A
roof catchment tank with a storage capacity of at least 16,200 + 20%
loss = 19,440 litres for fresh clean water for domestic use.
2) A
well in a riverbed or a pond that can supply 14,400 + 18,000 + 1,080 +
20% loss = 40,176 litres of unclean and, perhaps, saline water for the
livestock.
Many rainwater harvesting installations do not
perform as well as expected because of unsatisfactory gutters. It is
therefore important to give careful attention to the materials used, the
way the gutters are fabricated and the way they are installed. Ways of
fabricating and installing low-cost gutters are described below.
Semi-circular gutters
The best known gutter is semi-circular and made either of galvanized iron sheet or PVC. Gutters are laid in gutter brackets nailed onto facia-boards or in V-shaped tree branches nailed to the rafters with a gradient sloping towards the water tank. Bamboo and Sisal poles can also be used as gutters when split in two halves.
Simple and cheap gutter laid in tree branches (c) E. Nissen-Petersen, Kenya
Gutter suspended with a straight slope from a splash-guard nailed onto an uneven roof (c) E. Nissen-Petersen, Kenya
Splash-guards
A splash-guard, a strip of galvanized iron sheet, nailed onto the roof. (c) E. Nissen-Petersen, Kenya
They prevent rainwater from over-shooting gutters. They are made of strips of iron sheets bent at an angle and nailed onto the roof. Gutters are suspended with from the splash-guard using galvanized wires.
How to make and install Gutters with Splash-Guard
Marking an iron sheet into three stripes with a wire (c) E. Nissen-Petersen, Kenya
Cut galvanized plain iron sheets of gauge 26 or 28 into three strips, each being 200 cm long and 33.3 cm wide by marking the sheets with a thick wire, about 40 cm long, with each end having a sharp bend and a pointed end to scratch a line.
The distance between the two bends must be 33.3 cm in order to make equal width of the cut sheets.
The metal strips are bent over a U-shaped piece of iron and hammered into shape with a piece of wood or a mallet.
Bending the edge of an iron sheet (c) E. Nissen-Petersen, Kenya
The shape of the metal strips depends on whether they shall be splash-guards or gutters with one of the shapes shown below: namely the V-shaped gutter, the square gutter and the semi-circular gutter.
From left to right: Splash-guard; V-shaped gutter; square gutter and semi-circular gutter (c) E. Nissen-Petersen, Kenya
V-shaped gutter suspended from a splash-guard. (c) E. Nissen-Petersen, Kenya
Square gutter installed without splash-guard (c) E. Nissen-Petersen, Kenya
Gutters are fitted into hangers made of 3 mm galvanised wires that are bent over nails hammered into a piece of wood. (c) E. Nissen-Petersen, Kenya
Gutters are fitted into hangers made of 3 mm galvanised wires that are bent over nails hammered into a piece of wood.
Gutters fitted into hangers tied to a splash-guard nailed onto an uneven roof (c) E. Nissen-Petersen, Kenya
How to install Gutters
Gutters should be installed with a gradient of 10 cm depth for every 10 m length of a roof. This gradient of 1:100 will facilitate rain water running off the gutter with high velocity and no water will be wasted due to overflow.
The high velocity of the water will transport leaves and debris to the inlet sieve without blocking the gutter.
Splash-guard being nailed into a roof (c) E. Nissen-Petersen, Kenya
Water level in a hosepipe filled with water (c) E. Nissen-Petersen, Kenya
The two water levels in the hosepipe filled with water gives an exact horizontal level (c) E. Nissen-Petersen, Kenya
A gutter hanger is tied to the splash-guard with its bottom at the level of the water in the hosepipe. A second hanger is tied to the other end of the splash-guard near the tank.
This hanger is tied to the splash-guard with a slope 1:100 below the water level in the hosepipe. A gradient of 1:100 is distance is found by dividing the length of a roof with a factor of 100.
For example; if a roof is 20 m long, the hanger at the water tank must be 20 cm lower than the other hanger to get the desired gradient of 1:100.
The first gutter laid along a drawn string (c) E. Nissen-Petersen, Kenya
Gutter laid in hangers (c) E. Nissen-Petersen, Kenya
Two hangers are attached to the first gutter with one hanger at the middle and the other hanger at the end of the gutter.
Bitumen is smeared on the inner end of first gutter before a second gutter is laid into it and so on until the whole length of gutter is installed.
Other Types of Gutters
PVC pipes or Bamboo cut in half can be laid in timber or branches nailed onto the rafters. (c) E. Nissen-Petersen, Kenya
One side of galvanised and corrugated iron sheets can be nailed to the rafters, while the sheets are held in position by galvanized wires tied to the roofing nails. (c) E. Nissen-Petersen, Kenya
Baker R.L. (1998). Genetic resistance to endoparasites in sheep and goats. A review of genetic resistance to gastrointestinal nematode parasites in sheep and goats in the tropics and evidence for resistance in some sheep and goat breeds in sub-humid coastal Kenya. FAO/UNEP Animal Genetic Resources Information. 24:13-30.
Baker R.L., Mwamachi D.M., Audho J.O., Aduda E.O. and Thorpe W. (1999). Genetic resistance to gasto-intestinal
nematode parasites in Red Maasai, Dorper and Red Maasai X Dorper ewes in the sub-humid tropics. Animal Science (UK). 69(2):335-344.
Baker R.L., Mugambi J.M., Audho J.O., Carles A.B. and Thorpe W. (2002). Comparison of Red Maasai and Dorper sheep for resistance to gastro-intestinal nematode parasites: productivity and efficiency in a humid and a semi-arid environment in Kenya. Institut National de la Recherche Agronomique, Paris (France). Proceedings of the 7th world congress on genetics applied to livestock production. pp. 639-642.
Chemitei V.C.C. (1978). The sheep and goat production at various Research Stations in Kenya. Sheep and Goat Development Project, Ministry of Agriculture, Kenya. Technical Note No. 21. pp. 16.
DAGRIS, 2005: dagris
abc.net
The Organic Farmer’s Training leaflet No. 21
Wiener, Gerald (1994): Animal Breeding. The Tropical Agriculturist, Centre for Tropical Veterinary Medicine, University of Edinburg. Macmillan Publishers Limited, Between Towns Road, Oxford OX4 3PP. A Macmillan/CTA publication.
Almost every type of storage reservoir loses some of its stored water to evaporation, seepage or leakage.
Evaporation losses
Open water reservoirs, such as; tanks without roofs, ponds, earth dams and rock catchment dams, lose water due to evaporation. In hot and windy climates the evaporation rate may be over 3 mm per day which is equal to losing a depth of 0.9 metre of water in a month.
Water tanks and rock catchments should therefore be roofed but that is not feasible for ponds and earth dams.
Rainwater
stored in the voids between the sand particles of riverbeds is the most
economical water storage because up to 350 litres (35%) of water can be
extracted from 1 cubic metre of coarse sand while only about 10% of
water can be extracted from fine structured sand because its voids are
smaller.
In hot
regions without sandy riverbeds, rainwater can be stored in situ
between the voids between soil particles, e.g. downstream of ponds and
earth dams or in terraced land or in seasonal macro and micro irrigation
structures as described in “Water Storage”.
Seepage losses
In addition to evaporation loses, water stored in ponds and earth dams also lose water to seepage through the floor of these reservoirs.
Fortunately, some of the seepage loses from ponds and dams can be utilized by sinking a shallow well into a seepage area.
However, the combined losses of seepage and evaporation during long dry seasons usually result in the ponds and earth dams drying up except for very large earth dams.
The seepage loses through the floor of ponds and earth dams can be partly sealed by either:
1. Covering the floor with clayey soil or cow dung followed by cattle driven over the floor for compaction.
2. Waiting
for several rainy seasons to deposit layers of silt. Seepage losses
from sand in riverbeds can be avoided if proposed construction sites
with boulders and fractured rocks are rejected.
Leakage losses
Loss
of water from leaking water tanks, water pipes, water taps should be
prevented by using either bitumen paste or cement mortar.
With regard to disease treatments of organic sows or boars: if an organic sow or boar is treated with more than two courses of allopathic treatments in a year (or a piglet in a lifetime) it loses its organic status and must be reconverted or slaughtered as non-organic (vaccinations, treatments for parasites and statutory disease control measures are not taken into account).
Husbandry
Tail-docking, teeth-clipping and tethering are prohibited;
Artificial insemination and castration are allowed.
Although
regions with low and erratic rainfalls appear to be unsuitable for
rainwater harvesting, it has been proved many times that rainwater
harvesting is the most viable water supply system in arid and
semi-desert regions. Rainwater harvesting in dry regions is viable when
the following aspects are considered and applied:
Catchment areas are enlarged to increase the volume of run-off water.
Storage reservoirs are made large to store more water for longer periods.
Evaporation is minimized by roofing storage reservoirs.
Underground water storage in situ in the soil of farmland and sand of riverbeds.
The
following two examples show that rainwater harvesting is viable in
regions with little rainfall provided the catchment area is enlarged
accordingly:
Example 1
A roof with a catchment area of 100 square metres (m2.) and an annual rainfall of 800 millimetres (mm) can supply 72,000 litres of water (100 m2 roof x 800 mm rain = 80,000 litres minus 10% loss = 72,000 litres).
Example 2
A
roof in semi-desert regions with an annual rainfall of 200 mm has to be
4 times larger to supply an equal volume of 72,000 litres of water,
because the rainfall is only 1/4 of the 800 mm rain shown in Example 1.
To design successful rainwater harvesting systems, it is important to know:
1) How much rainwater falls on a catchment area.
2) How much of the rainwater runs off the catchment area.
If these two figures are not known, the storage capacity of a water reservoir and its spillways cannot be designed properly. The ruins of such improperly designed water projects can be witnessed in most parts of Africa.
The size of a catchment area is measured in square metres (m2) or in hectares (ha). 1 ha consists of 10,000 m2 which is equal to 2.47 acres. An acre is equal to 4,047 m2.
Farmers
measure their acreage of land by walking 70 paces, each pace with a
length of 3 feet, equal to 0.915 metre, around the four sides of a
square. 70 walking paces x 0.915 metres are equal to 64.05 metres.
When two sides of the square are multiplied with each other the result is 4,102 m2 (64.05 x 64.05), this is close to the actual area of 4,047 m2 for 1 acre.
When the size of a catchment area and the volume of rainwater falling on that catchment area have been found, the volume of rainwater that can be harvested is be calculated by multiplying the length with the horizontal width of the roof. For example: Length 20 m x horizontal width 5 m = 100 square metres
A sketch showing the length with the horizontal width of a double pitched roof (c) E. Nissen-Petersen, Kenya
Farmers may admit: “Okay, it has rained but it was only a drizzle”. Nevertheless, a drizzle of 10 mm rain can be sufficient to produce the required volume of water, if the catchment area is sufficiently large.
The relationship between rainfall and catchment area can be explained by the following example:
A
drizzle of 10 mm rain on10,000 m2 (1 hectare = 100 m x 100 m) area of a
roof, rock or tarmac road, it produces 100,000 litres of run-off water
minus about 20 % loss = 80,000 litres.
10 mm rain x 10,000 sq.m. minus 20% loss = 80,000 litres.
If
the same drizzle of 10 mm rains falls on a ten times larger catchment
area of 100,000 sq.m. (10 hectares = 100 m x 1,000 m,) it produces ten
times more water, namely 800,000 litres x 10 = 8,000,000 litres of water
= 8 million litres = 8,000 cu.m.
A pond for a catchment area (c) E. Nissen-Petersen, Kenya
Therefore, a drizzle of 10 mm rains may be
sufficient for harvesting a required volume of water, if the catchment
area is large enough. Huge volumes of water can thus be harvested from
e.g. roads because they have large and hard surface catchment areas.
For example: a drizzle of 10 mm rain on a road 6 metres wide and 1 km (1,000 metres) long road can supply the following volume of water: A 6 m x 1,000 m road x 10 mm minus 20% loss = 48,000 litres = 48 m3 of water.
Run-off water from a tarmac road. (c) E. Nissen-Petersen, Kenya
When designing a reservoir to hold water from a road catchment it is important that spillways can discharge safely the overflowing water during storms.
For
example: A rain storm of 75 mm falling on a 2 km long road produces
about: 75 mm rain x 6 m x 2,000 m road minus 20% = 720,000 litres = 720
m3 of water.
If the storage capacity, such as a pond is 500 m3, then the spillways must be capable of discharging 220 m3 of water (720 minus 500) in a few minutes or the pond will be damaged or perhaps washed away by the flood of incoming water.
Catchment of rainwater from roads is potentially the cheapest water source in dryland where there are no sandy riverbeds (luggahs or wadis).
Tarmacked roads produce more run-off water than dirt roads but the water may contain harmful tar components for people and livestock and should therefore be used for irrigation only.
There are two main types of storage, namely:
Storage in reservoirs, such as ponds, earth dams and tanks.
Storage in situ, that is in the voids between particles of soil and sand.
Although it is known that clouds can be seeded with chemicals to produce rains, the practice is expensive and unsustainable.
It should therefore be realized that rainfall varies from region to region and from one year to another beyond peoples’ manipulation and interference.
All fresh water comes from rains, including water in deep boreholes which originates from rains infiltrated into the underground thousands or millions of years ago.
Fresh water can only be obtained from four main sources; rainwater harvesting, shallow ground water, deep ground water and desalination of which the two latter options are too expensive to be discussed here.
The North-East monsoon coming from India brings rains to East Africa from October to December every year.
The South-East monsoon brings rains from March to June. Rain falls where and when the two monsoons meet.
The area of convergence is called the Inter-Tropical Convergence Zone. This zone follows the apparent movement of the sun, north and south, bringing rain in its wake.
But the pattern is influenced by mountain ranges, Lake Victoria and periodic westerly winds from the Atlantic.
The two monsoons bring an annual average of about 600 mm rainfall to the semi-arid eastern and northern parts of Kenya, while the highland zone at Mount Kenya has a mean average of 1,000 mm and the Lake Victoria zone has a mean average of 1,800 mm.
The rainfall pattern of East Africa is also presented as a map with different colours for the various average annual rainfalls.
